However, my question is: Should I replace the radiator for now and work my way up to the gaskets? Or am I wasting my time buying the radiator? OR is it possible to solder the cracked area?

You're not the first person with this dilemma - so you may save yourself some time & money by reading these threads, which were compiled ahead of time for you.

- Summary advice to provide users who suspect a major engine repair due to overheating (1) (2) (3) (4) & how to test an engine for a blown head gasket, cracked heads, a warped block, stripped head bolt threads, cam seizures, contaminated bearings, coolant hydrolock, or piston, ring, or valve damage (1) (2) & what are the major factors in deciding whether to rebuild the engine, replace the engine, or sell the car (1) & a DIY for replacing the I6 M54 head gasket (1) (2) & replacing the V8 M62TU head gasket (1) & why these engines are so prone to heat-related damage in the first place (1) & welding the crack between cylinder #3 and the water jacket on the exhaust side (1) & what engine swaps are most recommended (1) (2) (3) & where to obtain a new or rebuilt head (1) replacement short block or long block (1) (2) & how to lift & remove the engine (1) & the most recent real-world results from the last 50 people faced with similar blown engine problems from which this advice came from (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) (50)

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Please read the suggested threads, where the best always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need